1. Field
The present invention relates to frequency selective surfaces and, more particularly, to multiple layer frequency selective surfaces receiving electromagnetic radiation at oblique angles and performing electromagnetic conversion functions, such as polarization conversion, filtering, and frequency diplexing.
2. Description of Related Art
Frequency selective surfaces selectively pass electromagnetic radiation. An electromagnetic wave applied to a frequency selective surface (FSS) will be either passed through the surface or reflected off of the surface depending upon the electrical characteristics of the frequency selective surface and the frequency of the applied signal. A typical frequency selective surface comprises a doubly periodic array of identical conducting elements, or apertures in a conducting screen. Such a conventional surface is usually planar and formed by etching the array design from a metal clad dielectric substrate. These conventional frequency selective surfaces behave as filters with respect to incident electromagnetic waves with the particular frequency response being dependent on the array element type, the periodicity of the array and on the electrical properties and geometry of the surrounding dielectric and/or magnetic media. The periodicity is the distance between the centers of adjacent elements or between the centers of adjacent apertures.
One type of frequency selective surface known in the art comprises a continuous zigzag conductive grating supported on a thin dielectric sheet. Such a grating is typically known as a meander-line grating as in depicted in FIG. 1. In FIG. 1, the grating is shown as a parallel array 10 of meander line elements 14 oriented at 45 degrees from the horizontal and vertical. The meander-line grating can be designed to present specific inductive and capacitive susceptances to the TM and TE polarization of an electromagnetic wave incident on the grating. Hence, the meander-line grating can be used to control the polarization of an electromagnetic wave passing through the grating.
Many useful passive structures can be realized by using one or more frequency selective surfaces. Frequency diplexers, polarization converters, and filters can be realized by constructing multiple layer structures comprising layers of frequency selective surfaces spaced a certain distance apart (e.g., one-quarter wavelength of the operating frequency of the structure). A dielectric medium may be used to separate the frequency selective surfaces.
A general problem with multiple layer frequency selective surface structures lies in controlling the polarization mode coupling between the frequency selective surface layers. Most complex multiple layer structures are designed for normal incidence of electromagnetic radiation, since most applications require this. Such structures may be used with electromagnetic radiation at, or near, normal incidence, or, at most, within one or two planes of incidence, since the choice of polarization mode sets for multiple frequency selective surface layers that eliminate mode coupling is well-known in the art. Some multiple layer frequency selective surface structures have been shown to operate at up to 30 degrees off normal with the errors due to mode coupling effects limited to tolerable levels.
An example of a multiple layer frequency selective surface structure operable over a wide range of angles of incidence is disclosed by Hamman in U.S. Pat. No. 5,434,587, issued Jul. 18, 1995. Hamman describes a wide-angle polarizer comprising multiple layers of meander-line gratings. The meander-line gratings disclosed in Hamman are disposed parallel to each other, while the dielectric constants and thicknesses of the dielectric material surrounding and between the gratings are controlled to provide wide angle capability. However, wide-angle capability results in some deviation from perfect polarization conversion for any given oblique angle. Also, the polarization conversion capability of the Hamman device noticeably declines at large oblique angles of incidence, due to the inability to completely control polarization mode coupling.
Many applications, such as polarization converters for low profile satellite communication antennas, require performance optimization at oblique incident angles where polarization mode coupling may be strong. Hence, the particular advantages of specific individual complex frequency selective surfaces may be lost when combined into multiple layer structures, due to the mode coupling between layers.
It is therefore an object of this invention to provide multiple layer frequency selective surface structures operable at oblique incident angles by controlling mode coupling between layers of the structure. It is a further object of the present invention to select a suitable uncoupled mode set for each of the layers, and to provide a method of determining the orientation of each frequency selective surface with respect to the others to minimize coupling between the chosen polarization modes. In this way, the polarization conversion properties of the multilayer structure can be engineered to give the desired performance.